1. Field of the Invention
The present invention relates to a solenoid actuator for driving a movable portion of the solenoid actuator utilizing electromagnetic force. In particular, it relates to a filler for encapsulating the exciting coil of the actuator and means for isolating the filler from the movable portions of the actuator.
2. Description of the Related Art
As a solenoid actuator there is a wire dot print head which is used as a printing means for a printer.
A print head of this type is generally provided with armatures each connected to a print wire and a solenoid driving means for attracting the attraction of and releasing the attraction of the armature.
The solenoid driving means comprises a permanent magnet and an electromagnet for canceling the attraction of the permanent magnet. The electromagnet comprises a core and an exciting coil which is wound around a convex region of the core.
The exciting coil is fixed to the core by a filler such as an epoxy resin, silicone resin, or the like. The filler has a function of radiating heat generated in the exciting coil outside the solenoid driving means.
However, when the epoxy resin is used as the filler, the epoxy resin is chemically dissolved when the internal temperature of the print head exceeds 150.degree. C. during a printing operation whereby the dissolved epoxy corrodes B the covering member which is employed in the exciting coil, which generates the problem that the layer winding of the exciting coil is shorted.
If a silicone resin is used as the filler, it has a high viscosity and is very difficult to fill uniformly inside the solenoid actuator.
Since silicone resins can absorb low viscosity polymer oils, the resins absorb a lubricating oil which is applied between the core and the armature for preventing an abrasion caused by the striking between the core and the armature, which results in the abrasion thereof when a printing operation is carried on for a long period of time.
When the internal temperature of the print head rises to between around 100.degree. C. to 120.degree. C. due to the heat generated from the exciting coil for extended printing operations, silicon in the silicone resin reacts with oxygen whereby an extracted low polymer cyclohexane compound enters between the core and the armature, which hinders the armature from moving.
To solve the problems set forth above, there is a method to regulate the viscosity by mixing the low viscosity silicone oil with the silicone resin. However, such a problem remains unsolved in this method in that the absorption of the lubricating oil by the silicone resin increases.